The use of syngas from the gasification of carbonaceous feedstocks is of increasing interest for advanced energy generation and the production of alternative fuels and chemicals. Due to the interest in distributed power generation and liquid fuels production, hot syngas processing solutions have been developed and are gaining wide popularity.
The objective in gasification is to maximize the conversion of a carbonaceous feedstock into desirable syngas constituents (i.e., H2 and CO). This requires conversion of hydrocarbons, volatile organic compounds (VOC), and condensable semi-volatile organic compounds (SVOC) generated during gasification into syngas. Several different approaches have been and are being developed to affect this such as catalyst particles, catalyst packing, catalyst monoliths and catalyst coated or impregnated particulate filters.
Many investigators have developed catalysts to convert hydrocarbons, VOC and SVOC in steam reforming, autothermal reforming and partial oxidation modes of reactor operation. The key performance targets are conversion, yield, and selectivity while maintaining performance with negligible or minimal loss of activity. Potential factors that can lead to catalyst deactivation or degradation are: attrition or decrepitation of the particle or catalyst layer, sintering, agglomeration, coke formation, and poisoning due to feedstock-derived syngas impurities (H2S, COS, HCl, NH3, HCN, organic sulfur, chlorine and/or nitrogen compounds, metal vapors, metal carbonyls, etc.).
Some recent reviews include:    Gerber, M. A., “Review of Novel Catalysts for Biomass Tar cracking and methane Reforming”, Pacific Northwest National Laboratory Report PNNL-16950, October 2007.    Fang, H., Haibin, L., and Zengli, Z., “Advancements in Development of Chemical-Looping Combustion: A Review’, Intl. J. Ch. E., Volume 2009 (2009), Article ID 710515.    Kolb, G., “Fuel Processing: for fuel cells”, Technology & Engineering, 2008, 424 pages.    Yung, M. M., Jablonski, W. S., Magrini-Bair, K. A., “Review of Catalytic Conditioning of Biomass-derived Syngas”, Energy & Fuels, 23, pp 1874-1887, 2009.    Van Beurden, P., “On the Catalytic Aspects of Steam-Methane Reforming—A Literature Survey”, ECN-I-04-003 Report, December 2004.
Advanced catalysts which can operate under the adverse conditions of gasification are described in the following U.S. published patents and patent applications, whose contents are incorporated herein by reference: U.S. Pat. No. 7,329,691, U.S. Pat. No. 7,824,574B2, U.S. Pat. No. 8,105,973, US20060127656, US20070116639A1, US20080041766A1, US20090209412A1 and US20120046163.
US20060127656 to Gallo et al. discloses a novel catalytic membrane reactor for carrying out electrochemical reactions in a solid state. It comprises an organized assembly based on superposed layers of materials which have a perovskite-type crystallographic structure. This reactor is intended for the production of syngas by the oxidation of natural gas.
US20070116639A1 to Lomax et al. teaches the preparation of a catalyst that can be used for the production of hydrogen from hydrocarbon fuels in steam reforming processes; the catalyst contains an active noble metal, (e.g., at least one of Ir, Pt and Pd), on a catalyst support including at least one of monoclinic zirconia and an alkaline-earth metal hexaaluminate to exhibit improved activity, stability in both air and reducing atmospheres, and sulfur tolerance. Preferred reactor type is not indicated but the application seems to suggest a packed bed or fixed bed reactor.
US20080041766A1 to Giroux et al. teaches a method of reforming a sulfur containing hydrocarbon which involves contacting the sulfur containing hydrocarbon with a sulfur tolerant catalyst containing a non-sulfating carrier and one or more of a sulfur tolerant precious metal and non-precious metal compounds so that the sulfur tolerant catalyst adsorbs at least a portion of sulfur in the sulfur containing hydrocarbon and a low sulfur reformate is collected, and contacting the sulfur tolerant catalyst with an oxygen containing gas to convert at least a portion of adsorbed sulfur to a sulfur oxide that is desorbed from the sulfur tolerant catalyst. This invention is intended to be carried out in a simple reactor or a swing reactor but not a fluidized bed.
U.S. Pat. No. 7,824,574B2 to White et al. teaches a process for cyclic catalytic upgrading of chemical species using metal oxide materials. It includes two reactors, one for reduction and another for oxidation and the catalyst circulating between the two reactors.
US20090209412A1 to Parent et al. teaches a method of preparing a steam reforming catalyst characterized by improved resistance to attrition loss when used for cracking, reforming, water gas shift and gasification reactions on feedstock in a fluidized bed reactor. This reference discloses a preparation method comprising: fabricating the ceramic support particle, coating a ceramic support by adding an aqueous solution of a precursor salt of a metal selected from the group consisting of Ni, Pt, Pd, Ru, Rh, Cr, Co, Mn, Mg, K, La and Fe and mixtures thereof to the ceramic support and calcining the coated ceramic in air to convert the metal salts to metal oxides. This is specifically intended for fluid bed applications but is made in the form of spherical particles ranging in size from 100 to 1,000 microns by agglomerating catalyst support material. Typically the un-fired agglomerates are composed of catalyst support particles with an average size in the range of 0.3 to 10 microns, preferably in the range of 0.9 to 5 microns.
U.S. Pat. No. 8,105,973 to Basile et al. discloses a supported catalyst for producing H2 and/or CO from low molecular weight hydrocarbons. It employs the “form memory” concept to increase the interaction between the support and the active phase and thereby improve the stability of the resulting catalyst at high temperature.
US20120046163 to Ifrah et al. discloses a catalyst of the perovskite formula LaMO3, where M is at least one element selected from among iron, aluminum or manganese, in the form of particles dispersed on an alumina or aluminum oxyhydroxide substrate, wherein after calcination at 700 deg. C. for 4 h, the perovskite is in the form of a pure crystallographic phase, and in that the size of the perovskite particles did not exceed 15 nm.
One issue in using catalyst particles in a fluidized or entrained bed mode is attrition or decrepitation of the particle or catalyst layer and the need for make-up and the collection and disposal of the spent catalyst fines. In the case of catalyst packing, monoliths and catalyst particles employed in a fixed or packed bed mode, fouling or plugging by fine particles carried over in the syngas stream from the gasifier could adversely impact operation and performance. Catalyst deactivation or degradation due to poisoning by contaminants in the syngas stream such as sulfur compounds, coke formation, sintering, metal vapor deposition and agglomeration are common to both modes of operation. This requires catalyst regeneration via either continuous particle circulation through an external reactor or cyclic batch mode operation of two parallel reactors, one online for syngas treatment and the other in regeneration with its off-gas requiring further treatment. These tend to add complexity to equipment and operation and significantly increase the cost of syngas processing.
High temperature raw gas desulfurization has been carried out successfully by Li and Flytzani-Stephanapoulos using Cu—Cr—O and Cu—Ce—O oxides as regenerable sorbents. {Li, Z. and Flytzani-Stephanapoulos, M., “Cu—Cr—O and Cu—Ce—O Regenerable Oxide Sorbents for Hot Gas Desulfurization, Ind. Eng. Chem. Res., 36, pp 187-196, 1997}.
An alternate method of syngas cleanup and conditioning involves separation of particulate laden syngas through a filter, albeit a sintered porous metal or ceramic filter element, which may or may not have catalytic properties (see U.S. Pat. No. 8,007,688, and U.S. Pat. No. 6,077,490, and U.S. Pub. Pat. App. 2011/0058990 A1). Catalytic material may be coated, disposed, or impregnated into the candle filter element surface usually by sol gel or incipient wetness methods. An example is the high temperature removal of particulates and undesirable syngas constituents from biomass gasification syngas by use of catalytic candle filters {Ma et. al., Powder Technology, 180, 1-2, (2008)}. Methods have also been described to reduce levels of particulate and SVOC or VOC levels through filtration of the syngas through ceramic catalytic candle filters {Engelen et. al., Chem. Eng. Sci., 58, 3-6, (2003)}.
Three major drawbacks of the hot gas filtration method are: (i) the need to maintain low raw syngas flow or face velocity through the filter elements to avoid excessive pressure drop and achieve satisfactory particulate capture which makes the filter unit large and very expensive and impractical for gasifiers operating at low to moderate pressure (<75 psig); (ii) the propensity of the filter's pores and interstices to become clogged with particulate matter, either by carbon deposition, or SVOC condensation (this in turn affects the overall up-time, efficiency and ability of the catalyst to reduce levels of hydrocarbons, VOC, SVOC or other contaminants on a continuous basis); and, (iii) potential for catalyst deactivation due to poisoning by raw syngas contaminants or coking with a loss of performance with time. As such, there is a need for a more robust and economical method to improve raw syngas quality.